Correct Descriptor Semantics for Initialized Annotations
A shipped Meta Pyrefly contribution that stops annotation-only descriptor inference from misclassifying instance fields initialized in recognized methods. PR #4476 was imported into Meta's internal workflow and shipped upstream as commit 8387bfd.
#4476
public PR
8387bfd
upstream commit
2
files changed
+21 / −2
focused diff
01 · Engineering problem
The failure mode
Pyrefly could treat an annotated instance attribute as a class-installed descriptor whenever its type implemented __get__, even when the field was actually initialized on self inside __init__. That produced incorrect descriptor semantics for a valid instance-field pattern.
02 · Technical response
What changed
- 01Traced the behavior to DeclaredByAnnotation and its existing initialized_in_recognized_method signal.
- 02Used that signal only to skip descriptor construction for the annotated-and-method-initialized case.
- 03Preserved unrelated class-field visibility and constructor behavior.
- 04Added regression coverage for annotation plus __init__ assignment while retaining annotation-only and method-only controls.
03 · Verified outcome
Evidence-backed result
- ✓Meta imported the contribution into its internal development workflow.
- ✓The final change shipped upstream in facebook/pyrefly at commit 8387bfded3736532e1b6c1c124264711ea0c50bc.
- ✓mypy_primer reported no type-check result changes across its open-source corpus after the narrowed patch.
- ✓The target semantics were corrected without broad field-classification changes.
04 · Public record
Source evidence
Independent open-source engineering by Patrick Ribbsaeter. Company and project names identify the public repository and maintainer context only. No employment, client, vendor, or partnership relationship is implied unless explicitly stated.